The field of the invention is magnetic resonance imaging (MRI) and, in particular, a gradient coil for an MRI system and image reconstruction method for use with that gradient coil.
When human tissue is subjected to a polarizing magnetic field B.sub.0, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field but precess about it at a Larmor frequency dependent principally on the magnetic field strength. The precession produces a resonance (NMR) signal that may be detected and used to generate an image by imparting a spatially dependent phase and frequency to the spins through the use of superimposed and orthogonal gradient fields (G.sub.x, G.sub.y and G.sub.z).
In one common imaging sequence, gradient fields are employed, individually or together, to first select a slice volume in which the spins are coherent by generating a functional "select" gradient in conjunction with an RF excitation. Second, by means of a functional "frequency" gradient, the frequency of the spins within this slice is changed according to their location along the select gradient. Finally, along an axis orthogonal to that of the select and frequency gradients, a progressively increasing functional "phase" gradient is applied to vary the phase of the spins according to their location along that axis.
The time sequence of nuclear magnetic resonance (NMR) signals for a set of different phase encodings producing an array of data that when operated on by a two-dimensional Fourier transform yields an image of a slice through the patient. A basic overview of MRI image reconstruction is contained in the book "Magnetic Resonance Imaging, Principles and Applications" by D. N. Kean and M. A. Smith hereby incorporated by reference.
It is desirable to be able to produce gradient fields with fast switching ("slew rate") and with high spatial gradients ("steepness"). Higher gradient slew rate generally allows faster acquisitions of the necessary MRI signals. Higher gradient steepness increases the spatial resolution of the imaging process permitting smaller voxels of a patient to be discerned. Higher gradient amplitudes (accompanying greater gradient steepness) can also reduce the time required to obtain an MRI signal.
Generally, both the slew rate and amplitude of the gradients are limited, first, by the maximum voltage and current of the gradient amplifiers powering the gradient coils. Changing from a greater maximum to minimum voltage in a given time increases the current through the inductance of the gradient coil at a faster rate increasing the slew rate whereas higher currents produce generally higher amplitude in gradient fields which, for the fixed length of the gradient coil, produces a steeper gradient.
Slew rate and steepness are also limited for physiological reasons: too rapid changes in the gradient fields can induce a tingling sensation in the patient and cause involuntary muscle contraction. For this reason, the FDA has placed limits on the rate of change of the magnetic field to which a patient may be exposed in a magnetic resonance imaging machine. Given the practical requirements of short imaging time, a limitation in the rate of change of the gradient magnetic field effectively limits the maximum gradient steepness that can be obtained with a given gradient coil.